1. Field of the Invention
This invention relates in general to tools for running casing hangers in subsea wells, and in particular to a tool that utilizes pressure intensification through differential area pistons to set the packoff for a casing hanger.
2. Description of the Prior Art
The subsea well of the type concerned herein will have a wellhead supported on the subsea floor. One or more strings of casing will be lowered into the wellhead from the surface, each supported on a casing hanger. The casing hanger is a tubular member that is secured to the threaded upper end of the string of casing. The casing hanger lands on a landing shoulder in the wellhead, or on a previously installed casing hanger having larger diameter casing. Cement is pumped down the string of casing to flow back up the annulus around the string of casing. After the cement hardens, a packoff is positioned between the wellhead bore and an upper portion of the casing hanger. This seals the casing hanger annulus.
One type of packoff proposed utilizes a metal seal so as to avoid deterioration with time that may occur with elastomeric seals. Metal seals require a much higher force to set than elastomeric seals. Prior art running tools have employed various means to apply the downward force needed to set the packoff. Some prior art tools use rotation of the drill string to apply setting torque. It is difficult to achieve sufficient torque to generate the necessary forces for a metal packoff, because the running tool may be located more than a thousand feet below the water surface in deep water.
Other running tools and techniques shown in the patented art apply pressure to the annulus surrounding the drill string on which the running tool is suspended. The amount of annulus pressure is limited, however, to the pressure rating of the riser through which the drill string extends, which is normally not enough to set a metal packoff.
Higher pressures can be achieved by pumping through the drill string. However, this requires a running tool with some type of ports that are opened and closed from the surface. This is necessary because cement must first be pumped down the drill string. The ports may be open and closed by dropping a ball or dart. This requires a considerable amount of time, however, for the ball to reach the seat. Rig time is quite expensive. Another method employs raising and lowering the drill pipe and rotating in various manners to engage and disengage J-slots to open and close ports. This has a disadvantage of the pins for the J-slots wearing and not engaging properly.